lossless decoding: only process decoded row up to last_row

there's some subtle changes:
  - DecodeAlphaData() may be called with pos==end because we don't want
    to decode more data (there's none left), but because we want to apply
    process_func() to all the unprocessed pixels already decoded
  - last_row is exclusive and should be understood as 'up to last_row'. Can be misleading.
  - VP8LDecodeAlphaImageStream() was testing dec->last_pixel_ for completion,
    which was wrong because last_pixel_ is the last *decoded* pixel, not the
    last *processed* one. -> test now uses last_row_, as expected

Change-Id: I1fb04ba25cd7a4775db9e3deee3e2ae80f9c0a75
1 file changed